Thermally activated delayed fluorescence and mechanochromism in napthalimide-azatriangulenes

Abstract

Planarized donor and acceptor groups are promising building blocks for high-performance thermally activated delayed fluorescence emitters, as their rigidity minimizes non-radiative decay pathways. In this work, the planarized azatriangulene donors HMAT, OMAT, and SMAT are synthesized and incorporated into donor–acceptor (D–A) TADF materials with naphthalimide (NAI) acceptors. The relative strength of each donor is compared by photophysical and electrochemical measurements, finding that HMAT is the weakest donor of the series, while OMAT and SMAT have approximately equal strength. These emitters show green to orange emission with high photoluminescence quantum yields in toluene solution (0.75–1.0) and are TADF-active in doped films. In addition, each of the donor–acceptor dyads is found to display mechanochromic luminescence as neat powders. Analysis of X-ray diffraction data shows that the changes in optical properties upon mechanical stimulation are associated with a crystalline to amorphous transition, which may be due to disruption of weak intermolecular interactions in the crystalline powder.